The present invention relates to a laser oscillator and, more particularly, to a laser oscillator which is pumped by a discharge tube.
Such a laser oscillator is well known, for example, see "Flash Lamp Discharge and Laser Efficiency", APPLIED OPTICS, vol. 13, No. 10, October 1974, pp. 2300 to 2312.
In this type of laser oscillator, the laser element is optically excited or optically "pumped" by the light which is generated by the discharge tube. A predetermined high voltage obtained by an energy storage circuit is applied between both electrodes of the discharge tube. The discharge tube is also provided with trigger pulses of a predetermined pulse repetition frequency equal to that of the laser pulse, and the laser pulse is generated in response to each trigger pulse. Such a conventional laser oscillator, which operates on the basis of the pulse pumping or excitement by a discharge tube, however, suffers from the following disadvantage.
First of all, it is pointed out that this type of laser oscillator can exhibit only an extremely low laser efficiency which is expressed as the ratio of the laser energy to the exciting power. For instance, in case of a pulse pumped by YAG laser, the laser efficiency is as low as 1 to 2%. In consequence, associated equipment such as the energy storage circuit and the power supply circuit must be large in size. In case of portable laser equipment using batteries as the power source, the efficiency of power consumption is impractically low.
To explain in more detail, the gas contained in the discharge tube such as xenon or krypton, is transformed into a plasma state by the discharge current. The temperature of this plasma depends upon the discharge current density and the spectrum and the intensity of the light emitted from the plasma depend upon the plasma temperature. On the other hand, the laser element generates the laser based on the absorbed light having a wavelength peculiar to the element. In order to maximize the laser efficiency, therefore, it is necessary to select the plasma temperature such that the emission of light of the wavelength peculiar to the laser element is maximized. This means there exists a certain level of electric current density which is optimum. For instance, in the case of YAG laser, the maximum laser efficiency is attained when the discharge current density ranges generally between 4000 and 5000 A/cm.sup.2. This optimum range of discharge current density varies slightly depending on the kind of the gas in the discharge tube. Any discharge current density above or below this optimum range cannot provide sufficiently high energy efficiency of the laser. The electric current flowing through the discharge tube in the laser oscillator of the type described is pulsating, and the density of the discharged current is gradually increased from a low level to a maximum level and then decreases gradually. This means that the optimum discharge current density can be held only for a short time period, resulting in an impractically low laser efficiency.